U.S. patent number 7,876,505 [Application Number 12/186,671] was granted by the patent office on 2011-01-25 for objective lens simultaneously optimized for pupil ghosting, wavefront delivery and pupil imaging.
This patent grant is currently assigned to ITT Manufacturing Enterprises, Inc.. Invention is credited to Eugene G Olczak.
United States Patent |
7,876,505 |
Olczak |
January 25, 2011 |
Objective lens simultaneously optimized for pupil ghosting,
wavefront delivery and pupil imaging
Abstract
An objective lens includes multiple optical elements disposed
between a first end and a second end, each optical element oriented
along an optical axis. Each optical surface of the multiple optical
elements provides an angle of incidence to a marginal ray that is
above a minimum threshold angle. This threshold angle minimizes
pupil ghosts that may enter an interferometer. The objective lens
also optimizes wavefront delivery and pupil imaging onto an optical
surface under test.
Inventors: |
Olczak; Eugene G (Pittsford,
NY) |
Assignee: |
ITT Manufacturing Enterprises,
Inc. (Wilmington, DE)
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Family
ID: |
43479795 |
Appl.
No.: |
12/186,671 |
Filed: |
August 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61056552 |
May 28, 2008 |
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Current U.S.
Class: |
359/656;
359/370 |
Current CPC
Class: |
G02B
13/00 (20130101) |
Current International
Class: |
G02B
21/02 (20060101); G02B 21/00 (20060101) |
Field of
Search: |
;359/656,660,722,723,362,370 ;356/450 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Harrington; Alicia M
Attorney, Agent or Firm: RatnerPrestia
Government Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
The invention described herein was made in the performance of work
under NASA Contract No. NAS5-02200 and is subject to the provisions
of Section 305 of the National Aeronautics and Space Act of 1958
(42 U.S.C. .sctn.2457).
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority of U.S. Provisional Patent
Application Ser. No. 61/056,552 filed May 28, 2008.
Claims
What is claimed:
1. An objective lens comprising a plurality of optical elements
disposed between a first end and a second end, and each optical
element oriented along an optical axis, wherein each optical
surface of the plurality of optical elements provides an angle of
incidence to a marginal ray above a minimum threshold value, and
wherein the marginal rays are emitted from an exit port of a light
source, and the marginal rays are partially reflected from each
optical surface at an angle greater than a maximum angle, with
respect to the optical axis, for propagating light into an entrance
port of an optical measuring system.
2. The objective lens of claim 1 wherein the minimum threshold
value is approximately 8 degrees.
3. The objective lens of claim 1 wherein the angle of incidence is
a positive value, and the angle of incidence is defined with
respect to an intersecting line extending normally from each
optical surface of the plurality of optical elements.
4. The objective lens of claim 3 wherein each of the optical
surfaces provides an angle of reflection for the marginal rays that
is substantially equal to an angle of incidence of the same
marginal rays into a corresponding optical surface, and the angle
of incidence and the angle of reflection are measured with respect
to the intersecting line extending normally from the respective
optical surface.
5. The objective lens of claim 1 wherein the entrance port and the
exit port are disposed at the same plane, which is oriented
perpendicular to the optical axis, the marginal rays are emitted
from the exit port of an interferometer, and the light is
propagated into the entrance port of the same interferometer.
6. The objective lens of claim 1 wherein the partially reflected
marginal rays are reflected at an angle of reflection that is
greater than the maximum angle for propagating light into the
entrance port of the optical measuring system.
7. The objective lens of claim 1 wherein the objective lens has a
numerical aperture, and the minimum threshold value is greater than
0.8 times an inverse sine of the numerical aperture.
8. The objective lens of claim 1 wherein the plurality of optical
elements includes first and second doublets disposed between the
first and second ends, and an air gap is sandwiched between the
first and second doublets.
9. The objective lens of claim 1 wherein the plurality of optical
elements is configured to provide a conjugate image of an object at
an entrance port of an interferometer.
10. The objective lens of claim 1 wherein the plurality of optical
elements is configured to (a) receive substantially collimated
light at the first end, (b) transmit corresponding light from the
second end to a device under test to form an image, (c) receive
response light from the device under test at the second end, and
(d) transmit light from the first end to form an image conjugate
corresponding to the image formed at the device under test.
11. The objective lens of claim 1 including a null lens disposed
along the optical axis, at a location between the second end and a
device under test, and an interferometer configured to provide a
beam of light along the optical axis toward the first end.
12. The objective lens of claim 1 wherein an object placed at a
distance near infinity, along the optical axis, provides a response
light to the second end, and the plurality of optical elements is
configured to provide, from the first end, a finite conjugate image
of the object at an entrance port of an interferometer.
13. An optical measuring system comprising an interferometer
providing light, along an optical axis, from an exit port toward an
optical device under test, and receiving light along the optical
axis at an entrance port, an objective lens and a null lens
disposed along the optical axis between the interferometer and the
optical device under test, wherein the objective lens includes an
objective back disposed adjacent to the exit port of the
interferometer and an objective front disposed adjacent to the null
lens, the objective lens includes multiple optical surfaces
arranged along the optical axis, and marginal rays of the light
provided from the exit port of the interferometer have an angle of
incidence into each of the multiple optical surfaces, with respect
to an intersecting line extending normally from each optical
surface, that is greater than a minimum threshold value for
reducing reflected marginal rays entering the entrance port of the
interferometer, wherein the minimum threshold value is
approximately 8-9 degrees, and a maximum angle for propagating
reflected marginal rays into the entrance port of the
interferometer is approximately 2 degrees.
14. The optical measuring system of claim 13 wherein the objective
lens includes first and second doublets disposed between the
objective back and the objective front, and an air gap is
sandwiched between the first and second doublets.
15. The optical measuring system of claim 14 wherein each of the
first and second doublets includes three optical surfaces.
16. The optical measuring system of claim 13 wherein the objective
lens is configured to receive a first light from the exit port of
the interferometer and focus the first light at a distance away
from the objective front, the objective lens is configured to
receive a second light from the optical device under test, by way
of the null lens, and the objective lens is configured to provide
the received second light to the entrance port of the
interferometer.
17. The optical measuring system of claim 16 wherein the first
light forms a spot image on an optical surface of the device under
test, and the second light forms a conjugate image of the spot at
the entrance port of the interferometer.
18. The optical measuring system of claim 17 wherein the spot image
on the optical surface of the device under test is diffraction
limited.
Description
FIELD OF THE INVENTION
The invention pertains to optics and, more particularly, to an
objective lens and a system and method for using an objective
lens.
BACKGROUND OF THE INVENTION
An objective lens, sometimes referred to as a diverger, may be used
to transform a collimated beam into a diverging (or converging)
beam. In the field of interferometry, an objective lens may be used
with an interferometer to address a part under test. The objective
lens transforms a collimated beam from the interferometer into a
diverging (or converging) beam that fills the numerical aperture of
a part under test. The part may be directly addressed, as is
typical for a spherical surface, or addressed through a null lens
or other apparatus.
SUMMARY OF THE INVENTION
To meet this and other needs, and in view of its purposes, the
present invention provides an objective lens including a plurality
of optical elements disposed between first and second ends. Each
optical element is oriented along an optical axis. Each optical
surface of the plurality of optical elements provides an angle of
incidence to a marginal ray above a minimum threshold value. The
minimum threshold value is approximately 8 degrees.
The angle of incidence is a positive value, and is defined with
respect to an intersecting line extending normally from each
optical surface of the plurality of optical elements.
Each of the optical surfaces provides an angle of reflection for
the marginal rays that is substantially equal to an angle of
incidence of the same marginal rays into a corresponding optical
surface. The angle of incidence and the angle of reflection are
measured with respect to the intersecting line extending normally
from the respective optical surface.
The marginal rays are emitted from an exit port of a light source,
and are partially reflected from each optical surface at an angle
greater than a maximum angle, with respect to the optical axis, for
propagating light into an entrance port of an optical measuring
system. The entrance port and the exit port are disposed at the
same plane, which is oriented perpendicular to the optical axis.
The marginal rays are emitted from the exit port of an
interferometer, and the light is propagated into the entrance port
of the same interferometer.
The partially reflected marginal rays are reflected at an angle of
reflection that is greater than the maximum angle for propagating
light into the entrance port of the optical measuring system. The
maximum angle for propagating light into the entrance port of the
optical measuring system is approximately 2 degrees with respect to
a marginal ray propagating parallel to the optical axis.
The plurality of optical elements includes first and second
doublets disposed between the first and second ends. An air gap is
sandwiched between the first and second doublets. The plurality of
optical elements is configured to provide a conjugate image of an
object at an entrance port of an interferometer. The plurality of
optical elements is configured to (a) receive substantially
collimated light at the first end, (b) transmit corresponding light
from the second end to a device under test to form an image, (c)
receive response light from the device under test at the second
end, and (d) transmit light from the first end to form an image
conjugate corresponding to the image formed at the device under
test.
A null lens is disposed along the optical axis, at a location
between the second end and a device under test. An interferometer
is configured to provide a beam of light along the optical axis
toward the first end. An object placed at a distance near infinity,
along the optical axis, provides a response light to the second
end. The plurality of optical elements is configured to provide,
from the first end, a finite conjugate image of the object at an
entrance port of an interferometer.
Another embodiment of the present invention is an interferometer
providing light, along an optical axis, from an exit port toward an
optical device under test, and receiving light along the optical
axis at an entrance port. An objective lens and a null lens are
disposed along the optical axis between the interferometer and the
optical device under test. The objective lens includes an objective
back disposed adjacent to the exit port of the interferometer and
an objective front disposed adjacent to the null lens. The
objective lens includes multiple optical surfaces arranged along
the optical axis. Marginal rays of the light is provided from the
exit port of the interferometer have an angle of incidence into
each of the multiple optical surfaces, with respect to an
intersecting line extending normally from each optical surface,
that is greater than a minimum threshold value for reducing
reflected marginal rays entering the entrance port of the
interferometer.
The objective lens is configured to receive a first light from the
exit port of the interferometer and focus the first light at a
distance away from the objective front. The objective lens is
configured to receive a second light from the optical device under
test, by way of the null lens. The objective lens is configured to
provide the received second light to the entrance port of the
interferometer. The first light forms a spot image on an optical
surface of the device under test, and the second light forms a
conjugate image of the spot at the entrance port of the
interferometer. The spot image on the optical surface of the device
under test is diffraction limited.
BRIEF DESCRIPTION OF THE FIGURES
The invention may be understood from the following detailed
description when read in connection with the following figures:
FIG. 1A shows a central and marginal light rays as they pass
through an objective lens, in accordance with an embodiment of the
present invention.
FIG. 1B shows marginal rays passing sequentially, at an angle of
incidence, through each of the surfaces of the objective lens of
FIG. 1, in accordance with an embodiment of the present
invention.
FIG. 2 shows a block diagram of a wavefront measuring system in
accordance with an embodiment of the present invention.
FIGS. 3A and 3B illustrate the forward and return light paths,
respectively, in a wavefront measuring system, according to an
exemplary embodiment of the present invention.
FIG. 4 is a plot of root mean square (RMS) wavefront error for a
wavefront measuring system according to an exemplary embodiment of
the present invention.
FIG. 5 illustrates an objective lens according to another exemplary
embodiment of the present invention.
FIG. 6 illustrates light rays emitted from a light source and
passing through a nulling device, toward a mirror surface under
test, in accordance with an embodiment of the present
invention.
FIG. 7 is another plot of RMS wavefront error for a wavefront
measuring system according to an exemplary embodiment of the
present invention.
FIG. 8A is an illustration of marginal rays as they enter the
objective lens of FIG. 1A and are, respectively, reflected from
each of the surfaces of the objective lens, in accordance with an
embodiment of the present invention.
FIGS. 8B through 8G are illustrations of the same marginal rays
shown in FIG. 8A, reflected individually from each surface of the
objective lens.
FIGS. 9A through 9G are illustrations of marginal rays as they
enter another objective lens (different from FIG. 1A) and are,
respectively, reflected from each of the surfaces of the objective
lens, where one of the surfaces does not provide the advantages of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
An objective lens, sometimes referred to as a diverger, may be used
to transform a collimated beam into a diverging (or converging)
beam. In the field of interferometery, an objective lens may be
used with an interferometer to address a part under test. The
objective lens transforms a collimated beam from the interferometer
into a diverging (or converging) beam that fills the numerical
aperture of a part under test. The part may be directly addressed,
as is typical for a spherical surface, or addressed through a null
lens or other apparatus.
A typical objective lens is optimized to deliver a diffraction
limited beam to the device under test. However, an interferometer
or any pupil conjugate wavefront sensor must provide a pupil image
from incoming light for wavefront evaluation. The imaging
performance with respect to the pupil is not optimal in similar
prior art devices. For complex modern optical systems, however, the
fiducial and optical surface details that are provided by the
invention are advantageous. This is especially true, if the system
under test includes segmented optical components, such as a
segmented primary mirror.
Objective lenses may introduce objectionable back reflections (or
"ghosts") at the interferometer. These ghosts may produce visible
interference rings in the data that mask and generally degrade the
quality of the measurements.
The invention overcomes the limitations of the prior art by
providing an objective lens that has diffraction limited optical
performance at two sets of conjugates. The first conjugate provides
for delivery of a diffraction limited wavefront to a measuring
device, for example, in response to a substantially collimated
light. The collimated light may be received from the optical output
of an interferometer, from a single pass sensor, or from an object
at or near infinity, for example. The second conjugate provides
high resolution of a device or a part under test in the plane of
evaluation of the wavefront sensing instrument, for example. In
addition, the present invention reduces the impact of first
reflection ghosts at an interferometer pupil.
In addition to the above, the system may be optimized for finite
conjugate imaging of a mirror under test to the interferometer
(aperture stop at the front focus of the objective) and infinite
object beam delivery (aperture stop at the back focus of the
objective lens).
This invention differs from that disclosed in application Ser. No.
11/948,508 (Pupil Imaging Objective Lens) in that resolution is not
pushed to as high level though still much improved compared to a
lens with no optimization. The invention also includes the
important anti ghosting feature. application Ser. No. 11/948,508 is
incorporated herein by reference in its entirety.
As will be explained, for each surface in the objective lens, the
absolute value of the angle of incidence of the marginal rays is
maintained above a minimum threshold (this may be adjusted on a
surface by surface basis) for the infinite image conjugate. In an
exemplary embodiment, the minimum angle is approximately 8-9
degrees for all surfaces (that is, 8-9 degrees with respect to a
normal line to the surface of each lens). This has the effect of
producing divergent and out of focus reflections (that is,
reflections of 8-9 degrees with respect to the normal line from the
surface of each lens; or 16-18 degrees with respect to the angle of
incidence of the marginal ray).
The reflections from each surface of the lens result in low
intensities at the entrance to the interferometer. The bulk of the
reflections propagate at angles relative to the optical axis such
that only a small core of each reflection may reach the sensor
without obstruction (at an internal interferometer stop). In an
exemplary embodiment, the maximum angle, with respect to the
optical axis, that may propagate inside the interferometer and
reach the sensor is approximately 2 degrees. These effects (plus
antireflection coatings) contribute to achieving faint reflections
at the interferometer sensor.
The present invention may be used with all commercial
interferometers to reduce reflections from the objective lens into
the interferometer sensor.
An exemplary objective lens 100 is described with reference to FIG.
1A. The exemplary objective lens 100 includes a plurality of
optical lenses positioned along optical axis 114. The optical
lenses are designated in sequence from the back of objective lens
100 (defined as an end closer to aperture stop 110) to the front of
objective lens 100 (defined as an end closer to focal surface 112),
namely, lenses 102, 104, 106 and 108. An air gap, designated as
116, is formed between lenses 104 and 106.
Lenses 102 and 104 form a first doublet of the present invention
and lenses 106 and 108 form a second doublet of the present
invention. The first doublet includes three surfaces designated as
2, 3 and 4; the second doublet includes three other surfaces
designated as 5, 6 and 7. The surfaces 3 and 6 may be formed by
gluing (for example) lens 102 to lens 104 and lens 106 to lens 108,
respectively.
As shown in FIG. 1A, objective lens 100 may be optimized for
delivering an infinite conjugate. Three rays of light 118a, b and c
are shown entering the back of objective lens 100, which are
focused to pass through a conjugate point on focal surface 112 at
the front of the objective lens. The three rays of light, emitted
from an infinite point (not shown), become paraxial rays upon
passing through aperture stop 110, because they are substantially
parallel to optical axis 114.
In the exemplary embodiment, the optical elements 102, 104, 106 and
108 are made of glass. The invention encompasses, however, optical
elements that are reflective and optical elements that are made of
materials other than glass. For example, the optical elements may
include polycarbonate, zinc selenide (ZnSe), and silicon for
refractors, and aluminum, copper and beryllium for mirrors.
A first surface 110, which is also referred to herein as aperture
stop 110 (approximately co-located, for example, at an
interferometer entrance port), may be positioned behind the optical
elements of objective lens 100. The stop 110 is positioned a
distance d1 (i.e., the back focal distance) behind optical element
102 in this exemplary embodiment. A focal surface 112 is located a
distance d2 in front of objective lens 100. The image field of
objective lens 100 corresponds to the pupil size to be imaged.
Referring next to FIG. 1B, objective lens 100 is shown in greater
detail. Three rays of light are shown arriving from an
interferometer (not shown). The central, or axial ray is designated
as 118b and the two marginal rays are designated as 118a and 118c.
The marginal rays enter each lens surface, as shown from left to
right, forming angles of theta2, theta3, theta4, theta5, theta6 and
theta7, with respect to a normal line extending from each
respective lens surface. These angles are referred to herein as
angles of incidence.
The present invention is configured to provide an angle of
incidence to the marginal rays of light that is greater than a
minimum threshold. This minimum threshold may be, for example, 8-9
degrees. The angle of incidence is always positive.
Although not shown in FIG. 1B (but explained later with respect to
FIG. 8), reflection angles from each surface 2-7 are equal to the
angles of incidence to each surface 2-7. Thus, if the angles of
incidence are each at a minimum of 8-9 degrees than the angles of
reflection are each at a minimum of 8-9 degrees with respect to a
normal line extending from each lens surface. This, of course, is a
result of the Law of Reflection. The angles formed between a
marginal ray of incidence and a marginal ray of reflection is twice
the angle of incidence or the angle of reflection, namely, a
minimum threshold of 16-18 degrees.
Since these angles are much greater than the angle for illumination
that may propagate inside an interferometer to reach the sensor,
namely approximately 2 degrees with respect to the optical axis,
the present invention is effective in reducing ghost reflections
from the objective lens into the interferometer entrance port.
In this example, the F number (the ratio of the focal length
divided by the entrance pupil diameter) of the objective lens is 3.
As such, the marginal rays have an angle of incidence greater than
9 degrees at the objective lens front focus (with respect to the
optical axis). Thus, this example shows that the ghost reflections
are well managed, if the threshold angle is greater than 0.8 times
the marginal ray angle at the objective lens front focus. In other
words, the threshold angle should be greater than 0.8 times the
inverse sine of the numerical aperture of the objective lens.
A block diagram of an exemplary system 200 for directing light to a
device under test is shown in FIG. 2. The system 200 includes
objective lens 202 according to an exemplary embodiment of the
invention. The objective lens 202 has a first end 202a for
receiving light beam 214 from light source 206. The objective lens
202 directs a corresponding light beam 210 out from its second end
202b to the device under test 204. Exemplary embodiments of device
204 may be a mirror, an assembly of mirrors, deformable mirrors, a
telescope, an optical assembly to be aligned, or any combination
thereof. The light 208 returned from device 204 is received at
second end 202b of objective lens 202.
The objective lens 202 directs the corresponding light 212 from its
first end. As shown in FIG. 2, light 212 is not necessarily
directed back to light source 206. In the exemplary embodiment of
FIG. 2, light 212 is directed to wavefront sensor 216 (may also be
processor 216). Such wavefront sensor 216 may be an optical pattern
recognition device or an interferometer, for example.
In an exemplary embodiment, wavefront processor 216 processes
returned light 212 received from objective lens 202 to characterize
device 204 under test. For example, wavefront processor 216 may
process returned light 212 to identify surface details of the
device under test. In an exemplary embodiment, the device under
test may be a segmented mirror, including a plurality of mirror
segments, and wavefront sensor 216 may be used to provide
information regarding the orientation of the mirror segments. The
orientation information may be fed back (feedback path 218 shown in
phantom in FIG. 2) to device 204 for correcting or adjusting the
orientation of the mirror segments, in response to the identified
surface details.
Exemplary embodiments of the invention may include light 212
directed to the same unit that provides the source of light. For
example, source light 214 may be provided by an interferometer, and
light 212 directed from first end 202a of objective lens 202 may be
directed back to the interferometer.
Another exemplary system, designated as 300, for directing light to
a device under test, is shown in FIGS. 3A and 3B. The forward light
path is illustrated in FIG. 3A and the return light path is
illustrated in FIG. 3B. The system 300 includes objective lens 302,
according to an exemplary embodiment of the invention,
interferometer 306 and device under test 304. The device under test
in this exemplary embodiment is a segmented mirror. The objective
lens 302 directs light received from interferometer 306 to
segmented mirror 304 and directs light returned from segmented
mirror 304 back to interferometer 306.
With reference to FIG. 3A, objective lens 302 receives light 314
from interferometer 306 at first end 302a of objective lens 302.
The objective lens 302 directs corresponding light 310 from second
end 302b to segmented mirror 304. In an exemplary embodiment,
system 300 may optionally include a nulling device 316, or null
lens 316, as shown in phantom in FIGS. 3A and 3B. In an exemplary
embodiment, nulling device 316 is a nulling apparatus as described
in U.S. Pat. No. 7,336,370, titled "Optical Nulling Apparatus and
Method For Testing an Optical Surface" (issued Feb. 26, 2008),
which is hereby incorporated by reference in its entirety.
The nulling device 316 receives light from second end 302b of
objective lens 302 and directs such light 310 to the device under
test 304. In addition, the nulling device receives light 308
returning form device 304 and directs such light to second end 302b
of objective lens 302.
With reference to FIG. 3B, light 308, which is returned from the
device under test 304, is received at second end 302b of objective
lens 302. The objective lens 302 directs corresponding light 312
from its first end to form a pupil image at 322, which is seen by
interferometer 306. The entrance port of interferometer 306 is
designated as 322. The objective focal plane of objective lens 302
is designated as 320. The entrance port of interferometer 306 is
also referred to as the objective back of objective lens 302. The
objective focal plane of objective lens 302 is referred to as the
objective front (as shown in FIG. 1A).
The objective lens 302 of system 300 propagates light 308 backwards
from the device under test 304 to entrance pupil 322 of
interferometer 306. In an exemplary embodiment, system 300 includes
field stop 318, which may be placed at objective focal plane 320.
The field stop 318 may act as an aperture stop for the pupil
image.
In an exemplary embodiment, objective lens 302 of system 300, as
shown in FIGS. 3A and 3B, has an image entrance pupil diameter of 7
mm and includes the two doublets shown in FIGS. 1A and 1B. The
entrance pupil diameter value may be chosen to match the entrance
pupil size of interferometer 306. The exemplary system 300 may be
configured at F/3 and may be used in a center of curvature test
that employs null lens 316 and interferometer 306, with a
wavelength of 680 nm (ranging from 660-687 nm). The dimensions,
thicknesses, and spacing of the optical elements for an exemplary
objective lens 300 are shown in Table 1 below.
TABLE-US-00001 TABLE 1 Example of System/Prescription Data Dia-
Surface Type Radius Thickness Glass meter Conic Object Standard
Infinity Infinity 0 0 Stop Standard Infinity 19.49165 7 0 2
Standard 18.77147 2.953859 N- 11 0 LAK33 3 Standard -17.59841
2.270808 F15 11 0 4 Standard -60.26598 0.879023 11 0 5 Standard
-25.15344 2.999994 PSK54 11 0 6 Standard -8.743229 3.000074 SF6HT
11 0 7 Standard -20.82666 15.25284 11 0 Image Standard Infinity 1.5
0
With reference to Table 1, the back surface of lens element 102 in
FIG. 1A corresponds to surface 2 with a radius of 18.77147 mm, a
thickness of 2.953859 mm between surfaces 2 and 3, and a diameter
of 11 mm. The joint between element 102 and 104 corresponds to
surface 3 in Table 1, having a radius of -17.59841 mm, a thickness
of 2.270808 mm between surfaces 3 and 4, and a diameter of 11 mm.
The dimensions and spacing of the other optical elements 106 and
108 may similarly be determined, based on the exemplary
configuration specified in Table 1.
It will be appreciated that the optical data, shown in Table 1,
provide the radius of each of surfaces 2, 3, 4, 5, 6 and 7
(diagrammed in FIGS. 1A and 1B). Surface 2 has a positive radius
(convex surface) and each of the other surfaces has a negative
radius (concave surface). The thickness dimensions is the distance
in mm, between the surface involved and the next surface.
Accordingly, for example, the thickness 0.879023 mm for surface 4
is the distance in air gap 116 between surface 4 and surface 5.
Furthermore, the distance, 15.25284 mm, of surface 7 is the
distance between surface 7 and focal surface 112, namely distance
d2 (or the front focal distance).
Moreover, the distance of 19.49165 mm, shown for the stop surface
is the distance between stop aperture 110 and surface 2, namely
distance d1 (or the back distance to the entrance port of the
interferometer). The stop aperture 110 has a diameter of 7 mm.
Lastly, the optical elements each have a diameter of 11 mm. The
focal surface 112 has an image diameter of 1.5 mm. Also shown in
Table 1 are the various types of glass used for each surface.
FIG. 4 depicts the wavefront performance for objective lens 100
shown in FIGS. 1A and 1B, operating with an optic under test
positioned at infinity. The plots of RMS wavefront errors
illustrates that the objective lens is well corrected, with less
than one hundredth ( 1/100) wave RMS wavefront error at the edge of
the field (using a wavelength of light for which the lens is
designed). In an exemplary embodiment, the objective lens 100 is
configured to produce diffraction limited spots, where the RMS
wavefront error is less than about one tenth ( 1/10) of a wave in
the field of the lens. As used in relation to this exemplary
embodiment, the term "field" may be defined as an area or extent of
the object to be imaged by the objective lens, or as the area or
extent of the image that is produced by the objective lens.
An advantage of an objective lens according to an exemplary
embodiment of the invention may be understood by considering that
the pupil image of system 300 in FIGS. 3A and 3B is propagated
backwards from the device under test 304 (e.g., a segmented mirror)
to entrance pupil 322 of interferometer 306. In an exemplary
embodiment, field stop 318 placed at focal surface 320 may act as
the aperture spot for the pupil image. Although optic under test
304 is not typically at infinity, it is typically at a distance
that is large compared to the focal length of objective lens 302.
It will be appreciated that a similar field stop is focal plane 112
shown in FIG. 1A.
Referring next to FIG. 5, the same objective lens 100 is shown
operating as a finite conjugate. As shown, objective lens 100 is
optimized for delivering a finite conjugate image of device under
test 304 (FIG. 3B, for example) to stop aperture 110. The rays of
light are shown entering the back of objective lens 100 which are
then focused to pass through a conjugate point on focal plane 112,
at the front of the objective lens. In reverse, the rays may be
assumed to pass through focal plane 112 and next enter stop
aperture 110, as they arrive from a device under test, such as a
segmented mirror. Since the segmented mirror is not at infinity,
the rays passing through objective lens 100 are not focused at one
single point on focal plane 112, as they were in FIG. 1A. The rays
are, however, diffracted, or otherwise scattered toward focal plane
112 to fill an image of the interferometer stop that has a diameter
of approximately 1.5 mm at this surface, as shown in Table 1. It
will be appreciated that focal plane 112 may include the image spot
returned from a device under test that eventually forms the pupil
image at entrance port 322 of interferometer 306, shown in FIGS. 3A
and 3B.
Referring next to FIG. 6, operation of objective lens 302 is shown
as forming a finite conjugate at focal plane 112 (FIG. 5) for the
rays transmitted to a mirror surface under test, such as primary
mirror 304 (also shown in FIG. 3A). The small sketch in the bottom,
left portion of FIG. 6 depicts system 300 including primary mirror
304. The portion 600 represents the interferometer, the objective
lens and the null lens shown in FIGS. 3A and 3B.
An expanded view of portion 600 is shown at the upper, right
portion of FIG. 6. As shown, rays of light are emitted from
interferometer 306 to pass through objective lens 302 and null lens
316. The rays, after propagating through null lens 316, form a spot
image on primary mirror surface 304. Thus, objective lens 302 (or
objective lens 100) may be used to deliver a spot image to a device
under test, such as primary mirror 304. In the present invention,
an optical source spot is delivered through focal plane 112, which
produces a diffraction limited spot on the surface under test. In
reverse, a spot on the primary mirror surface is reflected back
through the null lens to deliver a spot at focal plane 112 of
objective lens 100. The spot is focused, as shown in FIG. 1A, or
may be accompanied by scattered light to fill the aperture, as
shown in FIG. 5. From focal plane 112, the spot passes through
objective lens 100 toward the entrance port 110 of an
interferometer.
FIG. 7 shows plots of the RMS wavefront error in waves versus the Y
field in millimeters, the latter representing the distance from the
center of the optical axis to the edge of the image impinged on the
PM surface. These plots illustrate that the objective lens is well
corrected with RMS wavefront errors at the edge of the field being
less than 0.025 waves.
Referring next to FIGS. 8A through 8G, single bounce ghost
reflections are shown for objective lens 100. As described above,
the present invention provides an objective lens that reflects a
marginal ray entering any surface of the objective lens by at least
approximately 8-9 degrees. FIG. 8A is a composite of light rays
118, including marginal rays 118a and 118c entering objective lens
100 and impinging upon all its surfaces designated by 2, 3, 4, 5, 6
and 7. The rays 118 enter from left to right, or from the objective
back to the is objective front (FIG. 1A), namely from surface 2
toward surface 7. As shown, all the reflected rays from the
surfaces, shown returning to the objective back, provide a
reflection angle that is greater than 8-9 degrees with respect to a
normal line extending from any respective reflection surface.
FIG. 8B shows surface 2 reflecting marginal rays 118a and 118c.
Surface 2 reflects the marginal rays by more than 8-9 degrees. FIG.
8C shows a reflection from surface 3. As shown, marginal rays 118a
and 118c pass through surface 2, next reflected from surface 3, and
then transmitted through surface 2. This reflection is also greater
than the minimum threshold of 8-9 degrees. Similarly, FIG. 8D shows
a reflection from surface 4. As shown, marginal rays 118a and 118c
pass through surfaces 2 and 3, and then are reflected from surface
4 toward the objective back of the lens. This reflection is also
greater than the minimum threshold of 8-9 degrees.
In a similar manner, FIGS. 8E, 8F and 8G show, respectively,
reflections from surfaces 5, 6 and 7. All these surfaces provide
reflections that are greater than the minimum threshold value of
8-9 degrees. None of these reflected rays can enter entrance port
110 (FIG. 1A) of an interferometer. Accordingly, the present
invention reduces or eliminates ghosts that are reflected back from
the objective lens into a wavefront measuring device.
Table 2 is another exemplary configuration of objective lens 100
including types of glass, and dimensional sizes for radius,
thickness and diameter. Table 2 includes six surfaces, identified
as 2 through 7, which correspond to surfaces 2-7 shown in FIG.
1A.
TABLE-US-00002 TABLE 2 Another Example of System/Prescription Data
Dia- Surface Type Radius Thickness Glass meter Conic Object
Standard Infinity Infinity 0 0 Stop Standard Infinity 15.64 7 0 2
Standard 12.99259 3.94 S- 12 0 BSL7 3 Standard -12.99259 3.6 S- 12
0 BSM2 4 Standard -10.50951 1.09 12 0 5 Standard -7.982652 2.83 S-
12 0 TIH53 6 Standard -18.11748 4 S- 12 0 BSM2 7 Standard -12.50354
14.93687 12 0 Image Standard Infinity 1.5 0
Referring next to FIGS. 9A through 9G, single bounce ghost
reflections are shown for objective lens 900. As described above,
the present invention provides an objective lens that reflects a
marginal ray entering any surface of the objective lens by at least
approximately 8-9 degrees. FIG. 9A, however, violates this rule. As
shown, marginal rays 118a and 118c enter objective lens 900
(including doublet 902 and 904; and doublet 906 and 908) and
impinge upon all its surfaces designated by 2, 3, 4, 5, 6 and 7.
The rays 118 enter from left to right, or from the objective back
to the objective front, namely from surface 2 toward surface 7. As
shown, most of the reflected rays from the surfaces, shown
returning to the objective back, provide a reflection angle that is
greater than 8-9 degrees with respect to a normal line extending
from any respective reflection surface. The exception to this rule,
however, is surface 6. This may be seen more clearly by examining
FIGS. 9B through 9G.
FIG. 9B shows surface 2 reflecting marginal rays 118a and 118c.
Surface 2 reflects the marginal rays by more than 8-9 degrees. FIG.
9C shows a reflection from surface 3. As shown, marginal rays 118a
and 118c pass through surface 2, next reflected from surface 3, and
then transmitted through surface 2. This reflection is also greater
than the minimum threshold of 8-9 degrees. Similarly, FIG. 9D shows
a reflection from surface 4. As shown, marginal rays 118a and 118c
pass through surfaces 2 and 3, and then are reflected from surface
4 toward the objective back of the lens. This reflection is also
greater than the minimum threshold of 8-9 degrees.
In a similar manner, FIGS. 9E and 9G show, respectively,
reflections from surfaces 5 and 7. All these surfaces provide
reflections that are greater than the minimum threshold value of
8-9 degrees. None of these reflected rays can enter entrance port
110 (FIG. 1A, for example) of an interferometer. FIG. 9F, however,
shows surface 6 providing reflected rays that focus at the pupil,
or entrance port of the interferometer. The angle of incidence at
surface 6 is 0.9 degrees. Accordingly, lens 900 does not eliminate
ghosts that are reflected back from the objective lens into a
wavefront measuring device.
Although the invention is illustrated and described herein with
reference to specific embodiments, the invention is not intended to
be limited to the details shown. Rather, various modifications may
be made in the details within the scope and range of equivalents of
the claims and without departing from the invention.
* * * * *